The invention relates to laryngeal mask airway devices for facilitating lung ventilation in an unconscious patient and to the coordinated use therewith of a reflectance oximeter for monitoring oxygenation in the blood as an indicator of current effectiveness of the lung ventilation.
Laryngeal-mask devices have been described in my U.S. Pat. Nos. 4,509,514 and 4,995,388, as well as in my copending U.S. patent applications, Ser. No. 919,289 filed Jul. 24, 1992, and Ser. No. 952,586 filed Sep. 28, 1992, which are incorporated herein by reference.
Such masks are artificial airway devices designed to facilitate lung ventilation in an unconscious patient by forming a low-pressure seal around the laryngeal inlet. A seal surrounds an appropriately shaped mask which fits into the lower pharynx and is attached to an airway tube which emerges from the mouth, as for connection to medical gas-supply tubing.
Such masks are artificial airway devices designed to facilitate lung ventilation in an unconscious patient by forming a low-pressure seal around the laryngeal inlet. A seal surrounds an appropriately shaped mask which fits into the lower pharynx and is attached to an airway tube which emerges from the mouth, as for connection to medical gas-supply tubing. More specifically the mask may comprise a plate with an inflatable ring having peripherally sealed engagement to the laryngeal inlet; and an elongate airway tube has a distal end that is sealed to the plate, for completing an externally accessible airway passage through the plate, for exclusive airway communication with the laryngeal inlet.
Pulse oximetry is a technique for non-invasively measuring the oxygen level in blood and has proved to be a very useful clinical tool as a means of continuously assessing the adequacy of ventilation, particularly in anaesthetized patients, for whom adequate oxygenation is a key concern. Most commercial pulse oximeters measure the signal when light of appropriate wavelengths is shone through a tissue such as a finger, and there are now more than 30 manufacturers of such devices. However, there are disadvantages to this technique, the most important being that the finger probe may easily become detached during use, with loss of signal, perhaps at a critical moment; also, since the device is peripheral in position, there is sometimes poor correlation between blood oxygenation in the finger, and oxygenation where it matters most--for example in the brain. Attempts to rectify this have resulted in development of probes for use on the ear, eyelid, and nose, with varying degrees of success.
Another approach has been to develop a related technique, reflectance oximetry, which has been described theoretically by Cui, Ostrander and Lee ("In Vivo Reflectance of Blood and Tissue as a Function of Light Wavelength", IEEE Transactions on Biomedical Engineering, 1990, vol. 37, no. 6: 632 to 639) and in certain practical applications.sup.1. The aim is to measure a signal reflected back from a tissue surface instead of having to rely on light transmission. This avoids the necessity of using peripheral tissues such as the finger, since tissue thickness is no longer a problem. The disadvantages of the reflectance oximeter are that accuracy is reduced, in that there may be problems with incident light, and it may be necessary to heat the skin in order to maximize blood flow in the skin, noting that skin is always at a lower temperature than blood. FNT .sup.1 See for example: FNT Cheng, Hopwood and Kay, "Forehead Pulse Oximetry Compared with Finger Pulse Oximetry and Arterial Blood Gas Measurement", Journal of Clinical Monitoring, 1988, vol. 4, no. 3: 223 to 226. FNT Mendelson and McGinn, "Skin Reflectance Pulse Oximetry: In Vivo Measurements from the Forearm and Calf", Journal of Clinical Monitoring, 1991, vol. 7, no. 1: 7 to 12. FNT Mendelson and Ochs, "Noninvasive Pulse Oximetry Utilizing Skin Reflectance Photoplethysmography", IEEE Transactions on Biomedical Engineering, 1988, vol. 35, no. 10: 798 to 805. FNT Johnson, Johnson, Fisher, Jobbings, Bannister, and Lilford, "Fetal Monitoring with Pulse Oximetry", British Journal of Obstetrics and Gynaecology, 1991, vol. 98: 36 to 41.